The cold weather has returned to the NC Piedmont, along with the usual humidity. On this night of roller-coaster seeing and pretty decent transparency I continued searching for Hickson groups and details within other galaxies.
NGC 925 – barred spiral in Triangulum:
With the release of Alvin Huey’s guide – Observing Extragalactic Objects Within Host Galaxies, it was time to revisit this galaxy in hopes of seeing some at least of the several HII regions identified. (There’s a print version as well.) I’d first seen it 2 years ago under very similar conditions as it turns out, so it was a good test. Back then just seeing galaxies was the goal. If you’ve not made use of Alvin’s guides, what are you waiting for? Just go to faintfuzzies.com
NGC 925 is classed as SB(s)d, with the southern arm being the more dominant of the two with tighter structure. The bar is also offset which, along with the dominant arm, gives it the characteristics of a Magellanic spiral. The galaxy is approximately 30 mly distant and is part of the gravitationally bound NGC 1023 group.
Observing: Transparency stayed pretty good throughout the night, between 9,800 – 10,200 ADU, but seeing was generally poor. It did bounce around from 1.4 – 4.10 a/sec, so between periods of atrocious skies there were some that were quite good. Just had to stay in the eyepiece and wait for them. An attempt to pick up more detail with the 2x PM @ 762x was not supported by the seeing conditions. The sky seemed brighter as well for some reason, and the SQM reading of 20.45 was indeed brighter than the average for GHRO of around 20.65 to 20.75. It was also fairly humid, limiting use of the hood to just a few minutes at a time as the eyepieces quickly fogged up.
The galaxy itself was easily found, marked by a thin bright triangle of stars to the southwest, and the bright core situated between two gently curved lines of stars that gradually converged to the east-southeast. The halo presented as a 2.5:1 oval, with the much brighter and elongated core area that was shifted more to the east side of the halo. No problem holding both directly. Averted vision showed that the halo extended further east and even more so to the west than direct vision showed. The western end of the galaxy showed some spreading and gradual bend to the north. The nebulosity had a mottled appearance. The eastern end of the galaxy was brighter than the west due to what looked to be the core extending into it. The nebulosity that continued to the east was not as broad as the western end and showed a minimal bend to the south.
45 minutes to an hour of looking teased out some of the HII regions I came to see. The designations used here are from page 71 of Huey’s Guide linked above. The eastern end of the galaxy, where I thought I was seeing a longer core, resolved into a pair of distinct bright round spots that corresponded to the HII regions [HK 83] 46/49 and 44. Seen in direct vision, these two knots were distinct and differentiated maybe 30% if the time.
To the west-southwest of the galaxy core is a pair of bright stars that form the narrow base of the triangle that marks the galaxy. Between the star closest to the galaxy and the galaxy’s core a faint and slightly elongated glow persisted, somewhat closer to the star than the galaxy. I wondered at first if I was seeing the star that’s in the area but was able to see both the star and the smudge at the same time when scanning around. This was the HII region [HK 83] 120/121.
A similar sighting occurred off the west end of the halo in the region of extended and mottled nebulosity. A brighter spot popped out from time to time, which I suspected of being one of the stars in a line that paralleled the brighter line of three to the north. But as with HK 83, I was able to see the star and the brighter round glow near it. There are a few HII regions in that broad area so it’s tough to be sure which of them I saw, but I suspect [KWB 2013] 41. There are three others on that end that I did not see, including one close to the western tip of the core.
The last HII knot seen was [HK 83] 42 to the south of the core’s eastern tip. A glow slightly brighter than the background haze popped in and out exactly between two faint stars to its east and west. [HK83] 16/18 on the opposite side of 925’s eastern tip was not seen.
Especially given the less-than-ideal conditions it was very gratifying to see these star-forming regions. I’d hoped for all of the ones identified on the page, but I’m happy with 5 for 9. Good training for a planned in-depth study of M33.
Sketching: white pastel pencils and white pastel powder on Strathmore 184 lb. black multimedia paper. A #2 Fibert laid down the halo and the core with a 10/0 mini Filbert. A 12/0 mini angular brush placed the HII knots.
Hickson 2 – compact galaxy group including UGCs 312, 314, 315 and MCG+1-2-18 in Pisces:
This compact group, from what I could find, is very tight indeed. Astronomy magazine relates that all four members span about 100,000 ly – less than the Milky Way’s diameter. That’s still a head-scratcher for me, as the galaxies would have to be tiny, showing very little dimension at their distance of around 180 million ly. The component members of HCG 2 are:
A: UCG 312 – a near edge-on SBd; mag. 13.9b
B: MCG+1-2-18 – an elliptical, mag. 14.5b
C: UGC 314 – SBc; mag. 15.0b
D: UGC 315 – SBb, mag. 16.0 (not seen)
Observing: A bright trapezoid of stars to the south-southeast is a nice marker for this group. Moving north leads to a distinct line of 3 stars aligned almost north-south. To the west of that line is another, much fainter line of 3 stars which runs along the northern edge of the group, marking members A and B. The two lines of stars converge towards the southeast. The last star in the fainter line forms a triangle with A and B, with B just off the end of the line. C and D are removed to the south.
B was seen first and was surprisingly bright once it was spotted. Bright and round, it showed a clear core, at times almost stellar, and distinct if uneven halo. This galaxy could be held direct at least 50% of the time.
A was the next brightest and presented as a 3:1 oval, elongated north-northeast to south-southwest. An elongated core was evident, though not as bright as B’s and certainly not stellar. This component could be held direct about 40% of the time.
C was a bear to nail down. Kept going back and forth between A&B and the area where C is located. A faint round glow puffed out after maybe 20-30 minutes, and it took another 10-15 minutes to confirm it. Seen in AV only, I couldn’t be sure that it had a winking stellar nucleus or if the intermittent flashes were from a star close by to the north. The glow was even, but incredibly faint, and no central brightening was observed. After more time I was pretty sure that the star was offset, so not a stellar nucleus. Not every sweep of the area turned up the glow – maybe 30% of the time - but successive views resulted in a slightly larger halo.
I looked for D most of the time I was scanning for C but with no results. Once C was spotted, I had a better idea of where it might pop up and spent more time looking for it specifically. After maybe another 20 minutes or so I called it quits. Both C and D are face-on spirals, so I expect their SB is awfully low. The night sky was brighter than average as well – 20.46-20.52 rather than the 20.60-20.75 I often get at GHRO. Alvin described it as “a very faint, small even surface brightness round patch.” Our transparency looks to have been the same, but his sky was certainly darker, so I think D just wasn’t going to happen from here. Having counted out the D component, I used the FOV that showed the two converging lines of stars and centered A and B while leaving C near the southern edge of view. D is out of the picture further SSE.
Three out of four isn’t bad, but at mags of 15.0 and 16.0 I was hoping for all 4 and didn’t expect C to be quite the pain it was. Lots of factors in play.
Sketching: white pastel pencils and white pastel powder on Strathmore 184 lb. black multimedia paper. All three halos were drawn with the 10/0 mini Filbert brush, as were the cores of A and B.
Thanks for looking everyone! Now it's wait for the moon to leave...
-b
Edited by bphaneuf, 06 December 2024 - 10:49 AM.
